Beaches experience erosion in response to energy resulting from waves that impinge on the shoreline. A variety of breakwater types have been previously used with varying degrees of success, to inhibit the deterioration of beaches. Most of the previous inventions in this area have been constructed in areas having relatively low tidal ranges. In regions where tidal ranges exceed one meter, the stage of the tide also plays an important role on the vertical distribution of wave energy on the beach face. The present invention employs a high-profile module designed specifically for regions with tidal ranges between one to three meters, but it is also suitable for deep deployment in low-tidal range regions.
In regions of relatively high tidal range, low-profile modules are often ineffective. If the devices are placed on the upper part of the beachface to shield the shore from waves at high water, the devices are left high-and-dry as the tide falls to low water level. If they are placed to intercept waves at low water, then they are too deep at high water to effectively shield the beach from incoming waves.
Since beaches are made of granular material, they are subject to change in direct response to the ability of the wind, waves and currents to transport the sediment. The process of erosion is an accounting problem related to sand transport by wind, waves and currents. Simply stated, when more beach material leaves a section of shore than it receives, the volume loss is described as erosion. When more beach material enters a section of shore than it loses, the volume gain is described as accretion. Since the capacity of a wave to transport sand is related to its size, then variations in wave size similarly relate to variations in the transport capacities of wave fields. Large waves, or strong wave-driven currents, have a greater capacity to transport beach material than small waves or weak wave-driven currents. By obstructing a portion of an incoming wave field, the capacity of the wave field to transport sediment is also diminished. The resultant is that less sand is removed from the beach than would be expected from the previously unobstructed waves. This is the main principal in the use of breakwaters for inhibiting erosion.
Some of the prior art has been directed toward trapping the littoral transport system. Others have been located further offshore to intercept wave energy before it reaches the shore. Much of the offshore systems have been composed of relatively small modules that are placed side-by-side and stacked to produce a submerged barrier parallel to the shoreline. Scour at the base of individual modules often causes them to shift, rotate forward, and/or sink into the seafloor. Stacks of multiple modules are massive, tend to sink into the seafloor rapidly and are difficult to remove or re-orient for breakwater modification or upgrade.
The cited patents represent an evolution of concepts that have provided partial solutions to some coastal areas of the world. Although each of these systems has provided valuable insights to the art, none have proven to be universally successful. For example, De Vilbiss (U.S. Pat. No. 1,753,776) describes a method of making a casing for making "filled concrete blocks". The concrete blocks may be used for revetment or levee work on river banks and can be filled with sand and gravel.
Campbell (U.S. Pat. No. 3,875,750) describes a modular unit for preventing and reversing erosion of waterfront land. The modules are elongated concrete blocks which are roughly triangular in cross-section, and have five peaks and four depressions. The central peak is the topmost, and two peaks are symmetrically located on either side of the central peak at progressively lower elevations.
Hubbard (U.S. Pat. No. 4,407,608) describes a modular structure for effecting deposits of fluid entrained alluvium. It is roughly triangular in cross-section, and has a smooth sloping rear face, and a concave surface along the front.
Weir (U.S. Pat. No. 4,498,805) describes a concrete breakwater module for shoreline protection. It is roughly triangular in cross-section, and has a vertical rear face, a large upwardly concave trough, and a sloping front wall. The rear wall is substantially higher than the front wall and the top and front walls have a variety of holes and passageways for redirection flow.
Schaaf et al (U.S. Pat. No. 4,722,598) describe a concrete module partially submerged to dissipate the energy of waves. The module is roughly triangular in cross-section, and has sloping seaward and rear faces. Openings on the front face lead to a series of passages that terminate in upwardly-directed openings on the rear face.
Brade (U.S. Pat. No. 4,776,725) describes an erosion control apparatus comprised of a plurality of interconnected members. Each of the members includes the equi-angularly disposed planar panels integrally coupled to a hub. One panel is vertically disposed and the other two rest on the seafloor.
Capron (U.S. Pat. No. 4,801,221) describes an "Oceanwheel Breakwater" which transfers lateral loads from the sea surface to the seafloor. The wall is composed of modular concrete cylinders held together by a tension spoke.
Mouton et al (U.S. Pat. No. 4,896,996) describes a series of low-profile beach cones for trapping sand on the beachface. Rows of beach cones are placed along the waters edge at low tide to eliminate the damaging effects of "undertow".
Atkinson et al (U.S. Pat. No. 5,011,328) describes a permeable breakwater constructed of precast concrete beams and plastic piping. The structure is roughly triangular in cross-section with an upward projecting (vertical) permeable wave wall.
Rauch (U.S. Pat. No. 5,120,156) describes a submerged breakwater comprised of a plurality of modules. The modules are roughly triangular in shape. The beachward and seaward faces are gently concave upward. The top of the modules have a short vertical wall with three open channels between the seaward and landward faces. The ends of the modules have interlocking members which allow construction along a continuous axial line.
Wheeler (U.S. Pat. No. 5,129,756) describes an apparatus for coastal erosion control using a massive seablock system. The blocks are large rectangular blocks that could be filled with sand, mud, shell or concrete rip rap. Each block has a massive concrete lid. The blocks are arranged along the shoreline in desired geometric patterns along coastal areas subject to erosion.
Creter (U.S. Pat. No. 5,238,326) discloses a concrete module partially submerged to dissipate the energy of waves. The module is roughly triangular in cross-section, and has sloping seaward and rear faces. The tops of the seaward and landward faces have transversely disposed concave cutouts that extend horizontally across the faces.